1. Field of the Invention
The present invention relates to a multi-channel video optical transmission system designed to transmit, through an optical fiber, a multi-channel video signal for use in a cable television system (CATV), and further to an optical transmitter and optical receiver constituting the same multi-channel video optical transmission system.
2. Description of the Related Art
FIG. 3 is a block diagram showing an example of configuration of a conventional multi-channel video optical transmission system. In FIG. 3, an optical transmitter 12 forming a transmitting side of the multi-channel video optical transmission system is made up of a pilot signal generating section 11 for generating a sine-wave signal with a given frequency as a pilot signal, a frequency modulator (which will be referred to hereinafter as an “FM modulator”) 2 for converting the pilot signal and an inputted multi-channel video signal, superimposed (multiplexed) on each other, into a frequency-modulated signal (which will be referred to hereinafter as an “FM signal) in batches, and a semiconductor laser device 3 for converting the FM signal into an optical signal and further for putting the optical signal out to an optical fiber 14.
In addition, an optical receiver 13 forming a receiving side of the multi-channel video optical transmission system is made up of a light-receiving device 4 for converting the optical signal, coming from the optical fiber 14, into an electric FM signal, an amplifier 5 for amplifying the FM signal outputted from the light-receiving device 4, a frequency demodulator (which will be referred to hereinafter as an “FM demodulator”) 6 for frequency-demodulating the FM signal outputted from the amplifier 5 into a superimposed signal comprising the multi-channel video signal and the pilot signal to put it out as an output signal from the optical receiver 13, a pilot level detecting circuit 7 for extracting only the pilot signal from the output signal from the FM demodulator 6, and an alarm circuit 8 for activating or giving an alarm when the pilot signal level detected by the pilot level detecting circuit 7 shows an abnormal value.
Still additionally, the multi-channel video optical transmission includes a tuner 9 for selecting a signal corresponding to one channel from the multi-channel video signal forming the output signal of the receive side FM demodulator 6 to convert it into a base band video signal and a TV monitor 10 for demodulating the base band video signal, outputted from the tuner 9 to display the demodulated signal.
Secondly, a description will be given hereinbelow of a concrete operation of this multi-channel video optical transmission system. In the optical transmitter 12 forming the transmitting side of the multi-channel video optical transmission system, a sine-wave signal with a given frequency, i.e., a pilot signal, outputted from the pilot signal generating section 11 is superimposed on an inputted multi-channel video signal, and this pilot signal superimposed multi-channel video signal is inputted to the FM modulator 2 to be converted into an FM signal. This FM signal is inputted to the semiconductor laser device 3 to be converted into an optical signal and then transmitted through the optical fiber 14 to the optical receiver 13 on the receive side.
In the optical receiver 13, the inputted optical signal is again converted into an electric FM signal in the light-receiving device 4 and then amplified in the amplifier 5. Following this, the amplified FM signal is inputted to the FM demodulator 6, thereby producing the original pilot signal superimposed multi-channel video signal corresponding to the signal before the input to the FM modulator 2.
In the meantime, the pilot level detecting circuit 7 fetches only the pilot signal to detect the occurrence or non-occurrence of an abnormality in pilot signal level, and if detecting the abnormality, outputs an abnormality detection signal to the alarm circuit 8 which in turn, activates an alarm. This permits the confirmation on the occurrence or non-occurrence of the abnormality in the optical transmitter 12, the optical receiver 13 or the optical fiber 14.
Moreover, the tuner 9 makes channel-selection from the multi-channel video signal outputted from the optical receiver 13, and the TV monitor 10 connected to the tuner 9 displays a desired channel video signal thereon.
However, in a case in which an input current (or input voltage)-vs-output frequency characteristic in the FM modulator 2 of the optical transmitter 12 shows non-linearity or in a case in which an input frequency-vs-output voltage characteristic in the FM demodulator 6 of the optical receiver 13 shows non-linearity, the above-mentioned configuration creates a problem in that intermodulation distortion (which will be referred to hereinafter as “distortion”) generally called CSO or CTB occurs in the multi-channel video signal outputted from the FM demodulator 6.
Likewise, in a case in which a group delay deviation occurs in the semiconductor laser device 3 of the optical transmitter 12 or in the light-receiving device 4 and amplifier 5 of the optical receiver 13, distortion occurs in the multi-channel video signal outputted from the FM demodulator 6, thus resulting in the occurrence of distortion in each channel band. In particular, in the case of a multi-channel video optical transmission system including a pilot signal, distortion occurs in the pilot signal and in the multi-channel video signal.
FIG. 4 is an illustration of a spectrum of a multi-channel video signal comprising a multi-channel video signal and a pilot signal superimposed on a frequency axis, and FIG. 5 is an enlarged spectrum illustration of one-channel video carrier and distortion in the multi-channel video signal shown in FIG. 4.
When the transmission of the multi-channel video signal shown in FIG. 4 takes place, in the output of the FM demodulator 6, distortion occurs at a frequency corresponding to the sum of or difference between each channel frequency and the pilot signal frequency. That is, as FIG. 5 shows, if the frequency of the distortion stemming from the pilot signal exists within the a video band of a video channel, when the picture is demodulated in the TV monitor 10, fringes (oblique fringes, horizontal fringes, vertical fringes or the like) appear in the picture, which interfere with the picture display. In the case shown in FIG. 5, distortion stemming from the pilot signal occurs at a frequency (fn−fp) existing within the video band on a video carrier with a frequency fa, and a fringe pattern appears on a video screen.